High volume, high speed production of metal products, for example sheet metal and pipe places demanding requirements on inspection techniques to identify flaws and characterize quality of the metal products. Presently, Faraday eddy current and ultrasonic equipment is used to identify flaws. The eddy current technique is limited to lower production speeds. At production speeds in excess of about 40 ft/sec, bandwidth limitations prevent accurate flaw detection with eddy current techniques. Ultrasonic equipment is limited because it cannot measure electrical resistivity, requires a couplant and is limited to low production speeds.
U.S. Pat. No. 3,881,151 uses an invariant magnetic field gradient and sensor for detection of flaws in moving electrically conductive objects such as tube, bar, wire, and wire rods. Detectable flaws include laps, cuts, seams and entrapped foreign materials. However, the '151 patent requires a magnetic field gradient and requires that the conductive material to be inspected be passed through a hollow cylindrical magnet. It further requires two sensors for flaw detection.
U.S. Pat. No. 2,519,367 to Gunn et al. is for a method and apparatus for detecting defects in objects as depicted in FIG. 1 (prior art). The apparatus or sensing unit 100 has a horseshoe magnet 102 embedded in insulating material 104 with the feet 105 of the horseshoe magnet exposed and placed adjacent to one side of the conductive material 106 to be inspected. A single sensing coil 108 is positioned between the legs of the horseshoe magnet 102 and is stationary with respect to the horseshoe magnet 102 by embedment within the insulating material 104 and is also placed adjacent the conductive material 106. Both the magnet 102 and sensing coil 108 are in relative motion with respect to the conductive material 106. An oscilloscope (not shown) receives a signal from the sensing coil 108 when a flaw (not shown) in the conductive material 106 passes by the apparatus 100.
The magnet 102 has a datum point 110 that is a point located on a center line 112 through a center of the magnet and substantially perpendicular to a material surface 114 and on a plane 116 defined by magnet surface(s) (feet 105) facing the material surface 114. The sensing coil 108 has a datum orientation with a longitudinal axis 118 of the sensing coil 108 substantially perpendicular to the material surface 114. In the prior art embodiment of FIG. 1, center line 112 and longitudinal axis 118 are co-linear. They are shown side by side in FIG. 1 for clarity.
Disadvantages of the prior art include (1) limited flaw size detection of flaws having a volume below about 1 mm.sup.3, (2) inability to detect subsurface flaws, and (3) inability to quantify flaw area or volume.